For ground-based astronomy, the quality of the
observing site is fundamental and this necessarily forces the astronomers to
place their telescopes in the most desolated and waste Earth’s places. In fact,
frontline optical and infrared observations require very good transparency
conditions, very low humidity, a high number of clear nights, steady and
non-turbulent winds and, of course, a dark sky. This has become even truer
during the last fifty years, during which the growth of cities and industries,
especially in Europe and North America, has added a new kind of environmental
contamination, which is usually referred to as light pollution.

Good
bye, blue sky

The problem has become indeed serious, so that
practically no professional astronomical observations are carried out from
continental Europe and all large European telescopes have been placed either in the Canary Islands or in some remote
places, usually in the northern and southern American continents.

To stop the light pollution and in order to diffuse
the awareness of the problem and the possible solutions to it, in 1988 the
International Dark-Sky Association was founded[1].
These noble tasks, though, are unfortunately very difficult to achieve and it
will probably take a long time before new lighting systems are put in place and
the bright halos surrounding our towns disappear from the night sky. It is very
sad to notice that the Milky Way is already a forgotten spectacle for many of
us. This is just another sign that we are loosing contact with Nature and its
beauty.

The effect of light pollution on astronomical
observations is in fact devastating. Street lamps do not illuminate just the
ground. Some radiation is reflected towards the sky and, in the most deprecated
cases, even directly sent upwards by lamps with a bad design. When this light
reaches the troposphere, it is reflected back to the Earth and it overwhelms
the natural night sky brightness, making the detection of faint and remote
astronomical sources very difficult, if not impossible.

The artificial illumination increases the sky
luminosities chiefly through emission lines of Mercury and Sodium vapor lamps,
which affect in particular the visual band, but have also intense emissions in
the blue and violet parts of the spectrum. Other lines are produced by special
elements (like rare earths) which are introduced in some lamps (like the
compact fluorescent lamps or CFL), in order to generate a spectrum which
resembles that of the Sun, to which the human eye is tuned and which allows a
perfect color recognition. Finally, there is a small contribution of
incandescence lights, which produce a continuum spectrum.

The effects of artificial light contamination are
clearly visible in the upper panel of Fig.2, where the night time spectrum
obtained with the 1.8m reflector of the Asiago Observatory (Italy) is shown. The
original scientific spectrum, which was aimed at studying a faint Supernova,
was obtained with the telescope pointing almost at zenith, in order to minimize
the impact of atmospheric extinction. Notwithstanding this precaution, the sky
spectrum, which has been extracted on an object-free part of the field of view,
shows very clear signs of artificial emission lines of Mercury and Sodium,
which are identified with red marks. In particular, the broad Sodium feature at
about 590nm is a clear imprint of high pressure lamps, which are the most
disturbing sources of light pollution.

The 1.8m Copernicus telescope is placed on the top
of MountEkar, at about 1300 meters
on the sea level. In spite of its relatively good location, the brightest
feature in its night sky spectrum is the Mercury line at 436nm (the only red
line in Fig.2), which is much brighter than the natural Oxygen line at 558nm,
the most intense feature emitted by the night sky.

Fig.2 –
Comparison between a night sky spectrum obtained in a light polluted
site (Asiago Astrophysical Observatory –
Italy, upper
panel) and a dark site (ESO-Paranal –
Chile, lower
panel). Spectral line identifications for the main features are traced
in red for artificial sources and in blue for the natural ones. The
emissions generated by street lighting are clearly visible, mainly in
the form of strong lines of Mercury and Sodium, which fall not only in
the visible range (500-600nm), but also in the in blue and violet
parts of the spectrum.

The presence of these lines causes the sky to
become artificially brighter, and it turns into a disaster for broad band
imaging. In a relatively protected site like MountEkar, in the B and V
filters the enhancement is more than a magnitude. But when one goes close to a
relatively big town, this degradation can reach two or more magnitudes, causing
many astronomical objects to be lost, not only for the visual observers, but
also for the evolved amateurs equipped with modern CCD cameras. A possible
solution to the problem, at least for photographic and digital imaging, is the
use of sky-suppression filters, which are designed to have a very low
transmission in the spectral regions centered on the main Mercury and Sodium
emission lines.

Of course, also professionals are affected by light
pollution, which makes deep imaging a very difficult task, since the augmented
sky background introduces an additional noise in the images, increasing the
detection threshold. The contamination is a bit less of a problem for
spectroscopy, since in that case one can still observe “in-between” the
emission lines. Unfortunately, the region where the high pressure Sodium broad
feature is sitting is also very interesting for several astrophysical fields.
If it is true that one can subtract the sky spectrum from its science spectrum,
this is not the case for the added noise, which cannot be removed and degrades
the data quality in an unrecoverable way, especially for low and medium
resolution spectral dispersions.

The artificial emission lines are so bright in
large towns that one can use them to achieve an accurate wavelength
calibration, without the need of obtaining dedicated exposures on special arc
lamps. A bitter advantage, though.

Fig.3 – Tracings of night sky spectra at a polluted
(red) and at a dark site (blue). Main line identifications for
artificial features are marked. The colored curves on the top of the figure are
the transmission functions of UBVR standard astronomical passbands. The light
pollution appears to be maximum in the V passband, which is very close to the
human eye sensitivity region. The B band is also severely contaminated, making
astrophotographer’s life quite difficult.

Moving
to the waste lands

As we have
seen, the problem is getting serious. Not only the amateur astronomer
who wants to observe with her telescope from the balcony is disturbed
by the light pollution. This is the case for the professionals
too. All the more. Good astronomical sites are hard to find, because a
long series of requirements, having to do with natural geographical
and meteorological characteristics need to be fulfilled. As a matter
of fact, nowadays professional astronomers perform their observations
mainly in remote places, either in dry and high altitude deserts or on
the top of high and isolated volcanoes. Nevertheless, even going in
the most abandoned places of the planet does not solve the problem
once and forever. In fact, once an observatory has been built, one has
to make sure that the growing human activities are not degrading the
excellent conditions for which the site has been chosen. For this
reason, the Commission n. 50 of the international Astronomical Union
for the protection of professional observatories has created a working
group to control the light pollution both at existing and potential
sites. The latter, in fact, could guest in the next 20 years, the
extremely large telescopes, reaching up to a hundred meters in
diameter, and for them one needs to plan everything well in advance,
including the prevention of contamination by human activities. For
this reason, all major observatories around the world have sky
brightness surveys, which serve both to detect any signs of light
pollution and to study long term trends, seasonal variations and so
on. This is true also for the largest optical and near-IR observing
facility in the world, the European Southern Observatory[2],
which represents the most important resource for all European
researchers. This international organization, originally founded in
1962, is supported by eleven countries and operates two large
observatories located in the Chilean Atacama Desert, La Silla and
Cerro Paranal, the latter hosting since 1998 the four 8.2m
telescopes.

Fig.4 – Cerro Paranal (Chile) as seen from the
base camp (credit European Southern Observatory).

In those sites the sky is absolutely dark. In
moonless nights the southern Milky Way is of an amazing beauty, the Magellanic Clouds
shine like atmospheric clouds and Omega Centauri is clearly visible to the
naked eye. No matter how many times you have seen it, it will always surprise
you, being one of the most fascinating Nature’s aspects one can experience.

The price astronomers have to pay for having such
excellent conditions is relatively high. Being far from any city, everything
needs to be brought there and this makes all buildings and infrastructures
pretty expensive. Also from the living conditions point of view things are not
easy. Humidity is extremely low (typical values range from 10% to 20%), there
is a constant wind blowing from a dominant direction (25-30 km per hour), the
elevation is quite high (more than 2500m at both observatories) and, besides
observing, there is not much one can do. But the reward which is given back in
terms of site quality is enormous. Seeing conditions are very good (0.7 arc
seconds on average), the number of clear nights is larger than 300 per year,
the transparency is very good and, of course, the sky is dark. As dark as it
can naturally get.

Fig.5 – The summit of Paranal Observatory with the
four 8.2m telescopes, which have been named using the Mapuche language: Antu
(the Sun), Kueyen (the Moon), Melipal (the Southern Cross) and Yepun (Venus). (Credit
European Southern Observatory).

The
night sky at Paranal

The four 8.2m ESO telescopes are
located on the top of Cerro Paranal in the Atacama
Desert in the northern part of Chile, one of
the driest areas on Earth. Cerro Paranal (2635 m) is at about 108 km south of Antofagasta (225,000
inhabitants), 280 km south-west from Calama (121,000 inhabitants) and 12 km
inland from the PacificCoast.This ensures that the astronomical
observations to be carried out there are not disturbed by adverse human
activities like dust and light from cities and roads. Nevertheless, for the
reasons we have outlined above, a systematic monitoring of the sky conditions
is mandatory in order to preserve the high site quality and to take appropriate
action, if the conditions are proven to deteriorate. Besides this, it also sets
the stage for the study of natural sky brightness oscillations, both on short
and long time scales, such as micro-auroral activity, seasonal and sunspot
cycle effects. For all these reasons ESO has started a systematic monitoring
campaign, which automatically extracts the sky background information from the
images that are obtained for scientific purposes. The results of this survey
confirmed that Paranal, similarly to La Silla and other Chilean sites, is one
of the darkest places on the planet and that there are no signs of artificial
contamination.

Nevertheless, even at dark sites like
Paranal, the night sky is not completely black.In fact, when one is observing
from the ground, there are several sources that contribute to its brightness, some
of which are of extra-terrestrial nature (e.g. unresolved stars/galaxies, diffuse
galactic background, zodiacal light) and others are due to atmospheric phenomena
(airglow and auroral activity in the upper Earth's atmosphere). While the
extra-terrestrial components vary only with the position on the sky and are
therefore predictable, the terrestrial ones are known to depend on a large
number of parameters (season, geographical position, solar cycle and so on)
which interact in a largely unpredictable way. In fact, airglow contributes
with a significant fraction to the optical global night sky emission (up to
50%) and hence its variations have a strong effect on the overall brightness,
which changes from passband to passband.

In the B filter, the spectrum is rather
featureless and it is characterized by the so called airglow pseudo-continuum,
which arises in layers at a height of about 90-100 km (mesopause) and extends all
the way from 400nm to 700nm. All visible emission features, which become particularly
marked below 400nm and largely dominate the U passband, are due to Herzberg and
Chamberlain bands of molecular Oxygen (O2).

The V passband is chiefly dominated by
Oxygen 558nm and to a lesser extent by Sodium D doublet 590nm and OI 630,636 nm
doublet. The relative contribution to the total flux of these three lines is 17%,
3% and 2%, respectively. Besides the aforementioned pseudo-continuum, several
Oxydryl (OH) Meinel vibration-rotation bands are also present in this spectral
window; in particular, there is one which is clearly visible on the red wing of
NaI D lines andanother two on the blue
wing of OI 630nm. All these features are known to be strongly variable and show
independent behavior. In fact, OI 558nm, which is generally the brightest
emission line in the optical sky spectrum, arises in layers at an altitude of
90 km, while OI 630,636 nm is produced at 250-300 km. The OH bands are emitted
by a layer at about 85 km, while the Na ID is generated at about 92 km, in the
so called Sodium-layer which is used by laser guide star adaptive optic
systems. In particular, OI 630,636 shows a marked and complex dependency on
geomagnetic latitude which turns into different typical line intensities at
different observatories and it is known to undergo abrupt intensity changes on
the time scale of hours.

In the R passband, besides the
contribution of NaI D and OI 630,636nm, which account for 3% and 10% of the
total flux in the spectrum, strong OH Meinel bands begin to appear, while the
pseudo-continuum remains constant. Finally, the I passband is dominated by OH Meinel
bands; the broad feature visible at 860-870nm, and marginally contributing to
the I flux, is due to molecular Oxygen.

Due to these phenomena, the sky is
darker in the blue (22.6 magnitudes per square arcsecond in B) and becomes
progressively brighter as one goes to the red (19.7 magnitudes per square
arcsecond in I). Then, when one enters the infra-red domain, the natural sky
background becomes more and more the dominant source of radiation, making
ground-based observations very difficult.

As many surveys have demonstrated, the
dark time sky brightness shows strong variations within the same night on the
time scales of tens of minutes to hours. This variation is commonly attributed
to airglow fluctuations. Moreover, as first pointed out by Lord Rayleigh, the
intensity of the OI 558nm line depends on the solar activity. Similar results
were found also for other emission lines (NaI D and OH). Moreover, B and V sky
brightness is well correlated with the 10.7 cm solar radio flux so that, during
a full sunspot cycle, it changes by about 60%, with the highest fluxes reached
during the maximum of solar activity. This means that the difference in sky
brightness between solar minima and maxima can reach about half a magnitude.

Fig.7 – Paranal’s southern horizon as seen from the
telescopes platform. The Southern Cross is just rising; its brightest star,
Alpha Crucis, is at an elevation of 6 degrees, while Beta Crucis is at only 2
degrees. The sky is so clear and the humidity so low that other stars are
visible almost until they disappear below the horizon (Credit L. Vanzi,
European Southern Observatory).

Besides the atmospheric airglow, there
are other natural sources. Among these, the dominant one is the Sun light
diffused by the dust distributed on the ecliptic plane, known as the zodiacal
light. This can contribute up to 50% of the total brightness if one is
observing at low ecliptic latitudes and, in general, it depends on the position
of the sky where one is pointing. In dark sites, like Paranal, this is clearly
visible also to the naked eye, as a diffused and wide cone, just after and
before the evening and morning twilights respectively.

The night sky must have impressed the
human beings since the beginning of their evolution, at the very origin of
Astronomy. Its beauty continues to astonish everybody who turns his eyes to the
heavens, no matter whether he or she is an astronomer or simply a lover of
Nature. Sadly, less and less people can nowadays enjoy this wonderful
experience, which is unfortunately becoming a privilege. One more reason to
love and protect it.

Fig.8 – The radio galaxy Centaurus A. The image was
obtained combining three B, V and R frames, about 5 minutes of exposure time
each, taken with FORS2 mounted at the 8.2m telescope Kueyen. The image quality
is 0.6 arcseconds (credit European Southern Observatory).

[1] More
information can be found at the IDA official web site www.darksky.org.

[2]
More information about ESO can be found at www.eso.org.
The site hosts a wide collection of astronomical pictures and other informative
material that can be freely downloaded.